If you would like to add your logo to your invoices, you would do so my editing your header, or footer templates.

First go to Setup > Firm Settings > Invoice Templates
(Note, if you are not an admin user, you may be restricted from this area)


HEADER 
Next to the header template, click Edit/Preview



You can remove the fields already there, if you plan on adding a custom logo/font.
When ready to add your image, click upload



Browse for the image you would like to add



Once inserted, you can edit the size of the image by dragging the corners of the image. 



Then save your template

To Edit your footer, simply edit the footer template, also under Setup > Firm Settings > Invoice Templates


These template changes come into effect when you Print/View your invoice, so to view your changes simply locate and invoice under Activities and go to Action > Print Invoice (Selected)

If you notice your header or footer is being cut off (you need a wider margin), you can increase your header and footer heights under Setup > Firm Settings > Invoice Templates